Extrusion pressure drop

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

We use variants of profile extrusion to produce tubing -with diameters of less then 1 mm and pipes with diameters exceeding 1 m, Wall thicknesses can vary from a few tens of micrometers up to several centimeters. Extruded window and door frames are more complex than pipes. Such profiles are largely hollow with internal ribs and fins that reinforce and divide the interior into two or more channels. We use solid rubber profiles in applications such as door seals and windshield wipers. We can produce foamed extrudates by incorporating a blowing agent, such as butane or carbon dioxide, into the polymer in the molten state. As the polymer exits the die, its internal pressure drops and the dissolved gas expands to form bubbles within the product. Examples of foamed extrudates include pipe insulation and automobile door gaskets. 

Numerous carriers were produced from different recipes and in different sizes and shapes in a 1 kg/min lab scale extruder and subsequently calcinated under different conditions in a furnace. The recipes included different types of diatomaceous earth, different types and amounts of binder and variation of the water content of the paste, which is a critical extrusion parameter. The shapes included among others rings, multiple-holed rings, finned rings, and trilobes, whereas normal cylindrical pellets were not made due to their well-known inferior activity to pressure drop ratio. 

Todd, D.B., Determining Pressure Drop in Extrusion, Plast. Compd., 17, 23 (1994)... 

The zeolite crystals are in the form of fine powders, which would cause a very high pressure drop in a packed bed. They have to be formed into granules of approximately 3 mm in diameter, by using clay binders, such as kaolinite and montmorillonite. The methods consist of pelletization with binders under pressure into short cylinders, wet extrusion with a fluid into continuous cylinders, and granulation by rolling with binders into spheres. They also need to be dehydrated and calcined to remove volatile components before use. 

The extrusion pressure could be further modulated by choice of dies or screens with appropriate L/D ratios. The pressure differential between points of die entry and exit due to viscosity for a Newtonian material moving within a cylinder is represented by Equation (4), derived from the Hagen-Poisuille expression for pressure drop in a pipe of constant diameter as (41)... 

Another parameter of interest in the extrusion process is screen pressure, as that has been shown to affect the size of the final pellets as shown in Figure 24 (31). The extruder can be fitted with a suitable pressure transducer to monitor the pressure drop associated with extrusion. One such... 

Through circulation dryers employ perforated or open screen bottom tray construction and have baffles that force the air through the bed. Superficial velocities of 150 ft/min are usual, with pressure drops of 1 in. or so of water. If it is not naturally granular, the material may be preformed by extrusion, pelleting, or briquetting so that it can be dried in this way. Drying rates are greater than in cross flow. Rates of 0.2-2 lb/(hr)(sqft tray area) and thermal efficiencies of 50% are realized. Table 9.7(d) has performance data. 

Estimation of power consumption and power efficiency of the use of moving moulding elements is important both from the theoretical and the practical point of view. It is also rather complicated. In the end of Sect. 2.2 we stated that the theoretical power consumed for extrusion of a melt through the head and that consumed by rotation (or vibration) of the core are strictly increasing functions of frequency (oo or 2) at a fixed specific pressure drop across the f = P/l (see Fig. 6). 

In its simplest form an extruder consists of a body with a rotating axis equipped with blades, the so-called worm which extrudes the clay through a die, an opening with a special shape and construction which varies according to product requirements. During this extrusion the pressure on the clay is gradually increased to a maximum value. Near the die the pressure drops to zero when the clay is extruded. 

The pressure drop calculated assuming the relationship Nu = 1.75 (Gz)1 3 for estimating h is smaller than the calculated AP, assuming isothermal flow. For the conditions depicted in Fig. 12.11, at T = 103 s 1 the isothermal pressure drop is about 30 % higher than the measured value. This fact must be taken into account in the design of extrusion dies, so that gross die overdesign can be avoided, as well as in capillary viscometry. 

These phenomena have been the subject of intensive study during the last 50 years and still represent major problems in polymer rheology. From a processing point of view they are very important, since melt fracture represents an upper limit to the rate of extrusion, and swelling and the large pressure drops must be accounted for in product considerations and in the design of the die and processing equipment. 

In the processing equipment of thermoplastics, many kinds of complicated flow configurations exist. The flow in a tapered die, for example, produces three components of deformation that due to flow from the reservoir into the die that due to telescopic shear within the die that to extensional flow within the die. These may be assumed separable and the separately calculated pressure drops may be added to give the total pressure drop. But in addition, each deformation mechanism contributes to post extrusion swelling. The components due to simple shear and extension at the die exit determine the swell ratio. 

A study was made of the ability of viscoelastic models to describe the measured material functions of unplasticised PVC during extrusion and to determine whether it was possible to reproduce the elastic properties of the large entrance pressure drop and small extrudate swell during the extrusion of PVC using a capillary rheometer. Models used were the Phan-Thien and Tanner model and the K-BKZ-Wagner model with a single exponential damping... 

In the extrusion die, the flow is driven simply by the pressure drop, as shown in Figure 13.23 and Figure 13.24. This type of flow can be analyzed relatively easUy, even for complicated rheological behavior. We will look at two geometries in detail flow between two flat plates and flow in the annular space between two cylinders. Both these geometries are used commonly to make floor tiles, cylindrical pipe pieces, and by cutting in half, semicylindrical roof tiles. 

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